mesa: Add core.h.

mesa: Add core.h.

core.h is the public header of core mesa.  GLX, WGL, and GLSL are
supposed to include this header file.  It should be noted that headers
included by core.h must not perform feature tests (#if FEATURE_xxx).
Otherwise, we cannot, for example, mix a FEATURE_ES2 libmesagallium.a
with a FEATURE_GL libglsl.a.